Galaxy S Duos has got ‘Dual SIM Always on’ feature, which lets the handset to keep both SIM cards active all the time. For ex, if you’re on a call using one SIM, the handset will still alert you when there’s a call coming in on the other SIM and you can forward that call to the active card and it also lets you to configure how incoming calls are managed while you’re using data on one of the SIM cards.